The present invention is directed to the electropolishing of aluminum and aluminum alloys. The electrobrightening and the electropolishing of aluminum and aluminum alloys are essentially selective-dissolution processes, in which the high points of the rough surface are attacked more rapidly than the depressions. In essence, these processes remove a surface skin of metal, thereby substantially entirely preventing any contamination of the surface skin with oxides of the base metal or with traces of residual inclusions, such as polishing and buffing compounds, while at the same time brightening the surface.
The prior art processes for electrobrightening of aluminum include the so-called Battelle process, which utilizes a mixture of 95% by volume phosphoric acid, five percent by volume sulphuric acid, and 12.5 grams per liter of chromic acid.
Another process reported in the literature (Aluminum, Vol. III, Fabrication and Finishing, American Society of Metals, 1967 edition, pgs. 634 and 635) utilizes an electrolyte containing 62.5% by volume phosphoric acid, and 37.5% by volume ethylene glycol monoethyl ether. This process is utilized for deburring applications, as well as for electrobrightening.
In all of these prior art brightening and deburring processes, the strongly acid electrolyte tends to attack the metal, thereby pitting the metal and reducing its brightness. Further, the ether compound utilized in the second composition above defined is volatile at the operating temperatures of the bath, which range from 170.degree. to 185.degree.F. The attack of the acid electrolyte on the aluminum being polished, is, of course, increased where appreciable amounts of other alloy metals are incorporated into the aluminum. For example, recently developed aluminum bumper alloys containing from about 4 to about 8% zinc may well be excessively attacked by the abovedefined acidic electrolytes.